1. Technical Field
The invention relates to the art of making gray cast iron and, more particularly, to the technology involving chemistry variations and heat treating variables for improving the characteristics of wear resistance and toughness.
2. DESCRIPTION OF THE PRIOR ART
Gray cast iron is the least expensive of all the cast metals. This is due to the type of raw materials used: pig iron, cast iron scrap, steel scrap, limestone, coke and air, all of which are relatively inexpensive. Most gray iron used commercially is used primarily in the as-cast condition. There has been little attention given to heat treatment and low alloying for gray cast irons through the years.
The general concensus of foundry operators in this country indicates that the chemical composition of gray cast iron should be about (using weight percentages here and throughout the description, except where noted otherwise): 2.0-4.0 carbon; 1.25-3.25 silicon; 0.75-1.25 manganese; 0.08-12 sulfur; and 0.07-0.20 phosphorus. Gray cast iron will have a microstructure consisting of a pearlite matrix (aggregate of ferrite and iron carbide) and suspended graphite flakes; suspended carbides are not present and not desired. If carbides are present, the iron becomes a different type such as mottled iron or white cast iron.
In the field of scuffing wear, gray cast iron (having poor impact strength) is usually used where,the required impact toughness in service is not severe. Such cast iron is resistant to scuffing wear because of the presence of a high amount of type A flake graphite and a coarse pearlite matrix. If gray cast iron is rapidly cooled, it may contain up to 50% by volume primary complex iron carbides (cementite) dispersed in the matrix. Unfortunately, however, these primary aggregated carbides impart brittleness to the cast iron which considerably limits its usefullness with reference to impact strength. While relegating gray cast iron to applications without the need for high impact strength, the main developmental effort has been to improve machining.
One attempt to provide greater machinability while retaining wear resistance of gray cast iron involves heat treating the as-cast iron to reduce hardness while retaining the pearlite microstructure (see U.S. Pat. No. 4,230,506). In this patent, the cast metal was alloyed with a combination of carbide forming agents such as chromium and vanadium, and with pearlite stabilizers such as nickel and copper. They were used in combination to provide a composite total in an amount of 2.25-3.85%; this is a considerable amount of alloying agent producing sizable aggregated carbides. The improvement in machinability was achieved by heat treating to an austenitizing temperature, slowing cooling over a period of 1-1/2 hours to a temperature level of 400.degree. F., and then air cooling. Slow cooling promoted the production of a ferritic matrix and reduced the hardness of such cast iron, making it more readily machinable. After machining, the iron was quenched to transform any retained austenite to martensite.
The problem with the '506 patent is that scuffing wear resistance may be retained at the sacrifice of toughness and strength characteristics, making it unsuitable for applications that require a high level for both of these characteristics. Without suspended carbide particles, such iron cannot offer improved abrasive wear resistance.
Similarly, in U.S. Pat. No. 3,384,515, the solution to the problem of machinability was to control heat treating to permit the promotion of complex iron carbides (aggregated cementite) while providing for incipient spheroidization of pearlite, thereby avoiding martensite and reducing the hardness of the material. The same problems remain with respect to lack of enhancement of toughness and strength characteristics and the absence of suspended carbide particles to enhance abrasive wear resistance.
In U.S. Pat. No. 2,885,284, an attempt was made to provide for an increase in both the abrasive wear as well as toughness of a variety of irons, including gray cast iron. The contribution of this patent is to incorporate high amounts of alloying ingredients in the form of aluminum and manganese to promote contrary characteristics. Aluminum is added in amounts greater than 1% to promote graphitization and manganese is added in amounts greater than 1.5% to promote carbide stabilization. There is no attempt to modify or introduce any unusual heat treating parameters; there is simply a reliance upon conventional processing and heat treating steps. This patent does not teach obtaining free or suspended carbides as a mechanism to improve abrasive wear resistance in gray cast iron (see column 3, line 22). The disclosure also admits, in column 2, lines 27-33, that the amount of aluminum or manganese that is incorporated will depend upon which characteristic is desired in the final product. To increase toughness, the carbon must be predominantly in the form of graphite promoted by the use of aluminum. To provide for increased hardness, the carbon must be predominantly in the form of carbides, which is promoted by the incorporation of manganese. This disclosure is an "either/or" teaching in that there is no suggestion that both of such characteristics can be achieved at a high level at the same time.
It is an object of this invention to provide a gray cast iron having both increased wear resistance and toughness which can be achieved by modification both in the chemistry and the heat treating techniques for gray cast iron.
It is another object to produce a gray cast iron which has a stronger matrix to retain and hold suspended graphite and carbides in a gray cast iron.
In addition, it is an object of this invention to provide the above type of gray cast iron which additionally has high tensile strength, high damping capacity, high heat conductivity, and more ductility than conventional cast irons.